Understanding the molecular basis for biochemical regulation is a fundamental problem in human medicine. Unregulated or uncoordinated activities generally lead to pathological conditions and, if not corrected, to uncontrolled proliferation or cell death. This proposal addresses this fundamental question with studies of the cAMP- and cGMP-dependent protein kinases. We wish to use small-angle scattering to study the conformations of the holoenzymes, their subunits and changes in conformation induced by binding the regulatory messengers cAMP and cGMP.